Terminology

Pre-emption means that (1) threads have priority levels, and (2) that a
higher priority thread can steal the processor from a currently-running
lower priority thread, and (3) it can do so "immediately" it needs to,
without waiting for some "safe" synchronisation point.
By these criteria, none of the current Haskell implementations are
pre-emptive, because no API assigns priorities to threads. So let's try
to avoid using the term.

Fairness can be defined by two main criteria:

No runnable process will be indefinitely delayed.

No thread can be blocked indefinitely on an MVar unless another thread holds the MVar indefinitely.

Cooperative scheduling describes an implementation in which it is the programmer's responsibility to insert context switch points in the code. An implementation that only provides cooperative scheduling cannot satisfy the fairness properties given above. A programmer who has access to all the code may be able to insert enough context switch points to satisfy fairness, but this isn't always possible.

Concurrent foreign call means a foreign call that should run concurrently with other Haskell threads. It is a requirement of "fairness" (above) that a foreign call should not prevent other threads from making progress indefinitely. Note that we used to use the term non-blocking foreign call here, but that lead to confusion with "foreign calls that block", which are foreign calls that may wait indefinitely for some resource, such as reading data from a socket. "foreign calls that block" are the main reason for wanting support for concurrent foreign calls.

Scheduling.
Most current implementations of
concurrency in Haskell use non-preemptive scheduling. That is, there are
explicit time points at which any one thread can yield, allowing other threads to run.
The only differences between implementations are in the granularity and positioning of the yield.

For Hugs, yield is inserted at certain I/O actions.

For ghc in non-SMP mode, yield is inserted after some count of allocations.

For yhc, yield is inserted after some count of bytecode instructions.

Arguably, Hugs has made the wrong choice from a fairness point of view. It would be possible to make Hugs yield more often, such as in IO-monad's bind operator, but even this wouldn't be quite enough for fairness, because a thread might hang indefinitely performing a non-IO computation. Yielding outside of the IO monad in Hugs doesn't seem possible without overhauling the concurrency implementation completely.

The notable exception to the above is GHC in SMP mode, which can run multiple Haskell threads simultaneously in separate OS threads, and hence simultaneously on multiple CPUs if available. This implementation is truly preemptive.

Levels

There are several levels of concurrency support which require sucessivly more
implementation support and imply more implementation overhead.

No Concurrency

The report says nothing about concurrency at all

Concurrent Friendly

Enough is specified to allow people to write completley portable programs and
libraries that while they may not depend on concurrency, will not break in the
presence of it. See "Thread-safety" above.

Concurrent Capabale

This would allow concurrency needing programs to be written, but perhaps not
as transparently as it curently is with GHC. This would include everything
needed to write concurrent programs without anything that would imply a
run-time or implementation overhead in the non-concurrent case.